The present invention relates to image display screens of the  less than  less than  flat screen  greater than  greater than  type. It relates more particularly to means used to facilitate and simplify operations for the positioning of the constituent elements of these screens.
There are different types of image display screens that come under the category of flat screens, for example plasma panels, liquid crystal displays, screens whose cells use a xe2x80x9cpoint effectxe2x80x9d phenomenon to produce an electron beam each or again light-emitting diode screens.
These different flat screens have the common feature of having a matrix structure: to each elementary dot of the displayed image there corresponds a cell (or even several cells in the case of color images) and each cell is defined substantially at the intersection of two or more arrays of electrodes. Consequently, the manufacture of these different types of flat screens entails the same difficult problem for each of them, a problem that lies in the difficulty of registering the different elements used to form a cell, namely the difficulty of positioning all these elements with respect to one another and in the same way for all the cells of the screen.
The following explanations, given with the example of plasma panels (abbreviated as PP hereinafter in the description), will provide for a clearer understanding of the importance of the above-mentioned problem of registration.
PPs work on the principle of an electrical discharge in gases. They generally comprise two insulating plates each bearing one or more arrays of electrodes and mutually demarcating a gas-filled space. The plates are assembled with respect to one another so that the arrays of electrodes are orthogonal. Each intersection of electrodes defines a cell to which there corresponds a gas space.
FIG. 1 shows the structure of an alternating color PP of the type using only two intersecting electrodes to define and control a cell as described especially in the French patent application published under No. 2 417 848.
The PP has two substrates or plates 2, 3. One of them is a front plate 2, namely the one that is on an observer""s side (not shown). It has a first array of electrodes called xe2x80x9crow electrodesxe2x80x9d of which only three electrodes Y1, Y2, Y3 are shown. The second plate 3 forms the rear plate. It is opposite the observer and therefore it is this plate that, preferably, is provided with elements that can prevent the transmission of light to the observer. It has a second array of electrodes called xe2x80x9ccolumn electrodesxe2x80x9d, of which only five electrodes X1 to X5 are shown. The two plates 2, 3 are made of the same material, generally glass. These two plates 2, 3 are designed to be joined to each other so that the arrays of row and column electrodes are orthogonal with respect to each other.
It is common practice that, in the front plate 2, as in the example shown, the row electrodes Y1 to Y3 should be separated from one another by black strips 4 (forming what is called a xe2x80x9cblack arrayxe2x80x9d) designed to improve the contrast between cells of different rows. The row electrodes Y1 to Y3 are covered with a layer 5 of dielectric material by which they are insulated from the gas.
On the rear plate 3, the column electrodes X1 to X5 are also covered with a layer 6 of dielectric material. The dielectric layer 6 is itself covered with layers forming strips 7, 8, 9 of luminophor materials respectively corresponding in the example to the colors green, red and blue. The luminophor strips 7, 8, 9 are positioned in parallel to the column electrodes X1 to X5, above these column electrodes from which they are separated by the dielectric layer 6. The rear plate 3 furthermore has separation barriers 11 that are parallel to the luminophor strips 7, 8, 9 and separate these strips from one another.
The PP is formed by the joining of the front and rear plates 2, 3. This joining sets up a matrix of cells. The cells are then defined each at the intersection between a row electrode Y1 to Y3 and a column electrode X1 to X5 with a pitch P1 parallel to the row electrodes that is given by the distance between the column electrodes and with a pitch P2 along the column electrodes that is given by the distance between the row electrodes. Each cell has a discharge zone whose section corresponds substantially to the facing surface of the two crossed electrodes. In each cell, the discharge into the gas generates electrical charges and in the case of a xe2x80x9calternatingxe2x80x9d PP, these charges collect at the dielectrics 5, 6 facing the row and column electrodes. In the example shown, this operation is obtained by means of recesses Ep1 to Epn made in the luminophor strips 7, 8, 9 substantially on the useful surfaces of the column electrodes X1 to X5, namely the surfaces of these electrodes that define the section of the discharge zone.
Thus, in the example shown, the intersections made by the first row electrode Y1 with the column electrodes X1 to X5 define a row of cells, each cell being represented by a recess: the first cell C1 is located at the first recess Ep1, the second cell C2 is located at the second recess Ep2 and so on and so forth until the fifth recess Ep5 which represents a fifth cell C5. The first, second and third recesses Ep1, Ep2, Ep3 are respectively located in a green luminophor strip 7, a red strip 8 and a blue strip 9. They thus correspond to monochromatic cells having three different colors which, in a set of three, may constitute a colored cell. Thus, for 1024 colored cells per row for example, the plate 3 must contain 1024 times per line the above-described structure. The column electrodes X1 to X5 have a width Lg1 of about 50 microns and their longitudinal axes are spaced out for example by 250 microns. This gives an idea of the difficulties of manufacture, especially for obtaining an accurate position of the recesses Ep1 to Epn.
The operating quality of the PP depends on the geometrical and dimensional characteristics of the cells, and hence on the quality of registration which is defined as the precision of the positioning, with respect to one another, of its elements such as the row electrodes and the column electrodes, the barriers 11 and the recesses Ep1 to Epn for which, in particular, the required precision of registration may be in the range of xc2x120 ppm (20 parts per million), i.e. for example 10 xcexcm.
Such precision is very difficult and hence very costly to obtain in the context of industrial-scale manufacture. Indeed, the manufacture on a plate 2, 3 of the different elements referred to here above makes use in particular of the technique of photographic masks used on photosensitive layers and/or techniques of silk-screen printing. For the rear plate 3 for example, after the array of column electrodes X1 to X5 has been formed and then the dielectric layer 6 has been deposited, the lumionophor strips 7, 8, 9 are deposited on this dielectric layer 6. Then, the recesses Ep1 to Epn are made in the luminophor strips, along with the separation barriers 11, with all the precision possible. The masks used to define the different patterns such as electrodes, recesses, etc. furthermore comprise, in a standard way, specific alignment or positioning patterns used to align elements to be made with those obtained at a previous level or stage of manufacture. It must be noted that the term xe2x80x9cmaskxe2x80x9d is used to designate both photographic masks and silk screens.
FIGS. 2a, 2b show alignment patterns Ma1, Ma2 of this kind corresponding in the example respectively to a mask 20 for the definition of the recesses Ep1 to Epn or a mask 21 for the definition of the column electrodes X1 to X5. These alignment patterns consist of registration patterns along the two axes X and Y and conventionally they are located outside a useful surface S1, S2 bearing the drawing (not shown) of the elements to be defined.
The alignment pattern Ma1 (FIG. 2a) has the general shape of a T formed by a horizontal aperture Oh and a vertical aperture Ov. FIG. 2b shows the alignment pattern Ma2: it comprises firstly three vertical reference marks R1, R2, R3 corresponding for example respectively to the column electrodes X1, X2, X3 and secondly a horizontal reference mark Rh. To define the position of the recesses with respect to one of the column electrodes, the electrode X2 for example, it is enough to place the mask 20 bearing the recesses so that the apertures Oh and Ov of the alignment pattern Ma1 are centered respectively on the horizontal reference mark Rh and the vertical reference mark R2.
Naturally, in order that the quality of the positioning of the recesses with respect to the electrodes should be the same for the entire useful surface S1, SZ these two masks 20, 21 should be perfectly matched.
The precision needed for the positioning of the recesses Ep1 to Epn with respect to the column electrode X1 to X5 is of the greatest importance. It may be required to within plus or minus some tens of ppm and of course this precision is required for the mask used to define elements. It is therefore not possible, when such precision is sought to use conventional masks for example of the type made with gelatin on a Mylar support whose cost is not very high, for masks of this type have dimensional variations of more than 10 ppm per 0xc2x0 C. as well as per percentile point of hygrometry. In addition to this, there is also imprecision due to tracing conditions.
The manufacturers therefore, in making these masks, have been led to use glass-based substrates with very high dimensional stability. However, these substrates have the drawbacks, in particular, of being limited in size and having a very high cost. Their use entails particularly heavy penalties when they are used to obtain insolation by contact for then, despite their high cost, they soon get damaged.
Another difficulty in making such registration arises out of the dimensional variations of the plate 2, 3 when it is subjected to heat treatment. The glass plates 2, 3 undergo a heat treatment effect that comes into play between the making of the electrode arrays and that of the luminophor strips 7, 8, 9 or separation barriers 11. The temperature reached is in the range of 580xc2x0 C, i.e. the softening point of glass. Upon return to the ambient temperature, the plates 2, 3 show major dimensional variations (in terms of shrinkage and compaction). These variations are difficult to take into account with a view to registration for they are not reproducible to a precision of within more than a few tens of ppm, especially with ordinary sodium-calcium type glass.
These explanations show the seriousness of the problem raised by the registration of the different constituent elements of an alternating PP including the registration made necessary by the assembling of the front and rear plates, in particular when these two plates each bear electrodes as is most usually the case. It must be noted that these problems exist in a manner that is quite similar for the other types of PP and more generally for all the flat image display screens, provided that, like the PP, they comprise a matrix of cells each controlled by means of at least two crossed electrodes.
The present invention is aimed at facilitating the registration of the different elements of the matrix structure display screens. It makes it possible to avoid the various drawbacks mentioned here above, and especially to overcome constraints imposed by differences in dimension between masks and/or between a mask and an already obtained level of elements.
To this end, the invention proposes to provide at least certain electrodes of at least one array with a shape such that it gives a dimensional latitude for example of about hundred ppm or even more and thus makes it possible to compensate for the dimensional differences which are detrimental to the quality of registration.
According to the invention, there is proposed an image display screen comprising a matrix of cells, at least two electrode arrays, the electrodes of one array being orthogonal to the electrodes of the other array and each cell corresponding to an intersection of electrodes, wherein at least one electrode array comprises so-called variable direction electrodes each positioned along a longitudinal axis and having a shape such that each one diverges from and then approaches its longitudinal axis to intersect it and pass alternately on either side of this axis and plot a repetitive pattern, the divergence shown by a variable direction electrode with respect to the longitudinal axis having an amplitude as a function of the position of the electrode with respect to a reference position, this divergence varying from one electrode to another.